Mobile broadband will continue to drive demands for higher overall traffic capacity and higher achievable end-user data rates in a wireless access network. Several scenarios in the future will require the end-user data rates of up to 10 Gbps in local areas. In order to meet these demands for very high system capacity and very high end-user data rates, wireless access networks will be deployed with distances between access nodes ranging from a few meters in indoor deployments up to roughly 50 m in outdoor deployments. That is, the wireless access networks will have a considerably higher infrastructure density than the densest networks of today. The wide transmission bandwidths needed to provide data rates up to 10 Gbps and above can likely only be obtained from spectrum allocations in the millimeter-wave band. High-gain beamforming, typically realized with array antennas, can be used to mitigate the increased pathloss at higher frequencies. Such the wireless access network is referred to as New Radio (NR) system.
Besides traditional licensed exclusive bands, the NR system is expected to be operating on unlicensed spectrum bands, such as 5 GHz and 60 GHz spectrums, especially for enterprise solutions. Thus the NR system needs to support coexistence, which enables spectrum sharing with different operators or other systems.
Listen-Before-Talk (LBT) channel access scheme (also referred to as “LBT scheme”) is the most flexible way to achieve the spectrum sharing. In the LBT scheme, a transmitter may detect whether an operating channel is idle or not firstly. If the operating channel is idle, the transmitter may transmit data on the operating channel LBT channel access scheme is a distributed mechanism, so that there are no needs to exchange information between different systems which may be more difficult.
Listen-After-Talk (LAT) channel access scheme (also referred to as “LAT scheme”) is provided to solve hidden terminal problem and exposed terminal problem occurred in directional communication. LAT scheme considers involving a receiver to sense the operating channel directly. Another motivation for LAT scheme is low interference environment (i.e. less collision) for naive direct transmission. For this reason, LAT scheme adopts opposite logic to LBT scheme. In LAT scheme, the transmitter sends data on the operating channel directly without detecting whether the operating channel is idle or not, and does not send data only when it is confirmed that the operating channel is occupied by interfering transmissions.
To compare different coexistence support mechanisms, simulations have been conducted to study both mean object user experience rate and 5% cell edge user experience rate under different traffic settings. The simulation results show that LBT scheme works much better than naive scheme (i.e. direct transmission without any coordination) and has similar performance with LAT scheme in 1 antenna case. This means LBT scheme is preferred and is widely used in current systems, e.g. Wi-Fi systems. However, in 100 antenna array case, LBT scheme has similar performance with naive scheme in low traffic case and worse performance than naive scheme in high traffic case. On the other hand, LAT scheme has much better performance than LBT scheme in terms of mean object user experience rate and 5% cell edge user experience rate.
Therefore LAT scheme performs much better than LBT scheme in beamforming case with massive Multi-Input Multi-Output (MIMO). However, NR systems or future wireless systems with high gain beamforming would not be able to rely solely on LAT scheme to have high performance characteristics in unlicensed spectrum bands due to the following reasons:                LAT scheme needs signaling coordination between different systems or cells. In other words, it could only be used for coexistence between the same kind of systems, e.g. intra-NR coexistence or when different systems exercise a coordination mechanism.        Some unlicensed spectrum regulations mandate the use of LBT scheme. For instance, in 5 GHz unlicensed spectrum in Europe, Japan, there is a regulatory requirement to implement LBT scheme.        Unlicensed spectrum typically requires different access technologies to coexist with one another. For instance, in 5 GHz unlicensed spectrum, Wi-Fi systems (and in future Licensed Assisted Access (LAA) systems) are deployed, which perform LBT scheme. Thus NR or other future wireless systems operating in 5 GHz unlicensed spectrum and performing LAT scheme would not be able to provide good coexistence with the existing wireless systems such as Wi-Fi and LAA systems. Similarly, in 60 GHz unlicensed spectrum, there will be wireless systems based on IEEE 802.11ad (in future IEEE 802.11ay), IEEE 802.15.3c, etc., and need to coexist with one another. Using LAT scheme does not fulfill the coexistence requirements adequately.        